1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor (hereinafter also referred to as a “photoreceptor” which is provided with an intermediate layer containing a polyamide resin having a specified structure and metal oxide microparticles, is reduced in the variation in sensitivity in a low-temperature and low-humidity condition and can attain good image characteristics even after being used repeatedly, and to an image forming apparatus provided with the photoreceptor.
More specifically, the present invention relates to an electrophotographic photoreceptor in which the intermediate layer contains a polyamide resin which is a condensate of a piperazine-based compound and an aliphatic di- or tri-carboxylic acid compound or a lower alkyl ester thereof or a mixture of these and metal oxide microparticles surface-treated with silicon dioxide anhydride, and also to an image forming apparatus provided with the photoreceptor.
2. Description of the Related Art
An electrophotographic system image forming apparatus (also referred to as an “electrophotographic apparatus”) utilizing electrophotographic techniques to form an image is used frequently for copying machines, printers, facsimile machines, and the like.
The photoreceptor used in an electrophotographic process is constituted by laminating a photosensitive layer containing a photoconductive material on a conductive support.
Although photoreceptors (referred to also as an “inorganic photoreceptor”) provided with a photosensitive layer containing an inorganic photoconductive material as its major component have been widely used so far, they have a drawback in any of heat resistance, storage stability, toxicities to human bodies and environments, sensitivity, durability, occurrence of image defects, productivity, production costs, and the like and no photoreceptor which satisfies all of these characteristics has been obtained.
Meanwhile, studies and development are now in progress concerning a photoreceptor (referred to also as an “organic photoreceptor”) provided with a photosensitive layer containing an organic photoconductive material as its major component and the organic photoreceptor is predominantly used in photoreceptor applications at present.
The organic photoreceptors have more advantages than inorganic photoreceptors in terms of toxicity, production cost and the degree of freedom of material design though they have some problems in terms of sensitivity, durability and environmental stability. For example, in the organic photoreceptor, the photosensitive layer can be formed by an easy and economical method typified by an dip coating method.
Examples proposed as the structure of the organic photoreceptor include a structure in which a monolayer type photosensitive layer obtained by dispersing a charge generating material and a charge transport material (also called a “charge transfer material”) in a binder resin (also called a “binding agent resin”) is laminated on a conductive support and a structure in which a laminate type photosensitive layer or a reverse laminate type photosensitive layer obtained by laminating a charge generating layer produced by dispersing a charge generating material in a binder resin and a charge transport layer produced by dispersing a charge transport material in a binder resin in this order or a reverse order on a conductive support.
Among these materials, a functional separation type photoreceptor provided with the laminate type photosensitive layer or the reverse laminate type photosensitive layer is widely put to practical use because it is excellent in electrophotographic characteristics and durability and has a high degree of freedom of material selection, allowing the characteristics of photoreceptors to be variously designed.
When a photosensitive layer is directly formed on a conductive support by application in the production of an organic photoreceptor, the photosensitive layer is easily affected by the surface of the conductive support and it is therefore difficult to form a uniform film having a uniform thickness. Therefore, there is a problem that the film thickness becomes uneven, causing various image defects and uneven densities.
In the organic photoreceptor provided with a photosensitive layer having a laminate structure, the conductive support is in direct contact with the charge generating layer. Therefore, when an electric field is applied by electrification, charges generate at a part of the charge generating material and the potential is locally dropped at a place near the charge generating material, giving rise to a problem of the generation of fogging on white paper or gray parts in reverse developing. This problem is particularly significant in a high-temperature and high-humidity environment.
As measures taken to deal with the aforementioned problems, it is conventionally known that it is effective to form a resin layer called an intermediate layer (also called an “undercoat layer”) between the conductive support and the photosensitive layer. For example, as the intermediate layer, a layer obtained by applying an alcohol-soluble polyimide resin to the support, followed by drying is proposed.
However, even if such an intermediate layer is formed, since these alcohol-soluble resins are largely varied in resistance according to circumstances such as temperature and humidity, there is a problem that the potential is largely changed with environmental variation, and also that there are image defects such as generation of black dots and memories and uneven densities, though good electric characteristics and image quality are obtained in a usual environment.
As the intermediate layer, there are proposed an intermediate layer containing titanium oxide microparticles coated with alumina (Japanese Unexamined Patent Publication No. 59-93453), an intermediate layer containing metal oxide particles surface-treated with a titanate-based coupling agent (Japanese Unexamined Patent Publication No. 4-172362) and, an intermediate layer containing metal oxide particles surface-treated with a silane compound (Japanese Unexamined Patent Publication No. 4-229872).
Also, a photoreceptor using a polyamide resin having a specified structure (Japanese Patent No. 2852432) and a photoreceptor using a polyamide resin having a small water absorption coefficient (Japanese Unexamined Patent Publication No. 2003-287914 and Japanese Patent No. 2841720) have been recently proposed.
However, the polyamide resin used for the intermediate layer has drawbacks that it is hardly soluble in a solvent, tends to cause uneven coating, deteriorates the dispersibility of titanium oxide, easily causing the solution to be clouded, and is increased in the accumulation amount of residual potential in a high-temperature and high-humidity condition, though it is affected little by the influence of environmental variations and has a small water absorption coefficient.
Therefore, the aforementioned proposals in the conventional techniques are insufficient as to the characteristics of the electrophotographic photoreceptor and it is desired to develop an electrophotographic photoreceptor having more excellent characteristics.
Therefore, it is an object of the present invention to provide a photoreceptor which is reduced in the variation in sensitivity in a low-temperature and low-humidity environment and can attain good image characteristics even after being used repeatedly, and an image forming apparatus provided with the photoreceptor.